A method for improving the filling power of cut and impregnated tobacco is demonstrated in U.K. Pat. No. 1,409,825. According to this method, a volume expansion of the impregnated tobacco may be achieved by inducing a rapid pressure change in the surrounding medium. However, such batch methods are rather uneconomical.
Alternative methods for improving the filling power of cut and impregnated tobacco involve heating it with the aid of a gaseous heating medium, after first impregnating the cut tobacco and water, or in the case of cut tobacco laminae, impregnating the tobacco with an organic or inorganic impregnating composition having a low boiling and/or evaporation temperature.
Various heating mediums have been employed in the prior art. For example, U.S. Pat. Nos. 3,409,022 and 3,409,023 discloses employing air or inert gas without any appreciable moisture content as the heating medium for volume expansion. However, in these methods the cut surfaces of the tobacco particles tend to form a crust preventing an elastic yielding of the outer cells or cell walls. This crust hinders the desired puffing of the tobacco.
Various contact times of the heating medium with tobacco during the volume expansion process have also been employed. For example, U.S. Pat. No. 3,357,436 specifies an extraordinarily extended contact period and employs several drying towers in the process. This results in a minimal volume increase of about 2-3%.
Conversely, U.S. Pat. No. 3,734,104 and DE Pat. No. 2253882 disclose using a contact period of only 0.5 to no more than 3 seconds in a gaseous vapor-containing heating medium in which cut and moistened tobacco stems are rapidly dried down to a moisture content of 6%. These methods result in an initial filling power increase of about 50%.
The heating medium employed for conveying and drying the tobacco has a velocity of about 40 meters per second. In order to achieve the desired contact period, or dwell time, this high velocity necessitates excessively long tubing systems. This results in a considerable pressure drop and correspondingly high energy requirements.
These types of systems also result in considerable waste since it is impossible to avoid fracturing of the tobacco particularly at the bends of the tubing systems as well as on separating the tobacco from the heating medium. The tobacco which is extremely dry and very brittle undergoes a considerable loss of quality.
In addition, prior to further processing, the tobacco's moisture content has to be raised to the customary value of 12-13%, and the initially achieved filling power improvement is effectively reduced unless extensive measures are taken to avoid this occurrence.
Similarly, in European Patent Application No. 0029588, the impregnated tobacco is expanded and dried by means of a fast flowing heating medium within a tubing system having 90.degree. bends and employing a maximum contact time of three seconds prior to separation from the heating medium in a separator apparatus. Likewise, in this method, it is scarcely possible to avoid embrittlement of the tobacco, a large proportion of waste, and a loss of quality.
Another method for carrying out the volume expansion is disclosed in U.S. Pat. No. 3,881,498. Here, the tobacco is agitated in a vibrating fluidized bed. The heating medium enters the bed through perforations. However, the flow velocity of the gas stream is limited by the floating capability of the tobacco particles, whereby the puffing effect is diminished. Furthermore, channeling may occur in the fluidized bed of tobacco particles, leading to irregular moisture distribution and non-uniform puffing.
In all of these prior art methods, the initial moisture content of the tobacco lies above 35%. The volume expansion is conducted in a single process step under practically constant conditions. Cyclones are employed for separating the air from the tobacco and/or for uniform drying. The final moisture content of the tobacco following such drastic processing conditions lies at about 2%, since the extreme puffing conditions are also maintained during drying. This results in the final quality of the tobacco being impaired and a large amount of waste.
An attempted improvement over these systems is described in U.S. Pat. No. 4,044,780 in which a tube is provided with gas inlets for conveying the tobacco in a stream of warm, moist air. Upstream of the outlet end, the tube is provided with a restriction effective to accelerate the flow and to create increased turbulence. This leads to better heat transfer during the drying of the tobacco but also results in the tobacco particles being subjected to increased fracturing due to turbulence and flow reversals under increased velocity while the tobacco is in a substantially dry state. This reduces the filling power improvement and creates a large proportion of dust.